85.3 Friday, Jan. 6 Coral bleaching as an adaptive mechanism facilitating transmission of algal symbionts to the next generation of coral host. MATZ, Mikhail V.; University of Texas at Austin email@example.com
High seawater temperatures cause corals to expel their endosymbiotic algae (zooxanthellae), resulting in “coral bleaching” that in severe cases may lead to coral death. The adaptive bleaching hypothesis (ABH) proposed by Buddemeier and Fautin (1993) suggested that corals bleach to make space for more adaptive exogenous zooxanthellae strains. However, numerous studies revealed that adult corals are unable to trade their symbionts for a new strain not originally present within the coral. Here, we explore an alternative possibility: that bleaching facilitates zooxanthellae transmission to the next generation of host. This inverted adaptive bleaching hypothesis (iABH) is based on three observations: (1) corals experience minor bleaching every summer, (2) zooxanthellae expelled during bleaching are viable, and (3) young aposymbiotic corals recruit to reefs at about the same time. Importantly, iABH predicts that evolution of the high bleaching resistance may be maladaptive from the zooxanthellae standpoint. To test the iABH premises, we let aposymbiotic Acropora millepora recruits to receive inflow from heated and unheated jars containing branches of three adult “donor” corals, containing different zooxanthellae clades, C1, C2, and D. The recruits successfully acquired the corresponding donor’s zooxanthellae, which is the first evidence of transmission of expelled zooxanthellae to recruits. Contrarily to the original expectation, heat stress diminished the rate of transmission of both C1 and D zooxanthellae. However, clade C2 was appreciably transmitted only from the heat-stressed donor. We hypothesize that clade C2 conforms to the iABH by relying on temperature cues and bleaching physiology to achieve transmission, while clades C1 and D may be transmitted via continuous trickle out of the host.